To record information in multiple layers of an optical information recording medium, in recent years, research has been made on a method for causing a recording material contained in the optical information recording medium to produce an optical change using a multi-photon absorption reaction such as two-photon absorption (see, for example, Patent Literature 1). Similar to a widely-used conventional optical information recording medium with a single recording layer, according to an optical information recording medium utilizing a multi-photon absorption reaction, a change in the refractive index of a recording material at a recording region and the thickness of the recording layer are determined, in consideration of the fact that reflected lights reflected at both upper and lower interfaces of the recording layer interfere with each other upon reading out the information (this is called interference effect), such that the difference between the reflectivity at a recorded portion and the reflectivity at a non-recorded portion (i.e., difference in the ratio of light beams which are reflected at the upper and lower interfaces of the recording layer and interfere with each other, and thereafter return to an optical pickup) is large. In an information recording medium disclosed in Patent Literature 1, consideration is made on the relationship between the film thickness and the reflectivity as shown in FIG. 2 of this literature, and the paragraph [0062] suggests that the thickness of the recording layer is preferably around λ/4n , where λ is the wavelength of the readout beam, and n is the refractive index of the recording layer, or even thinner to have a thickness of around 5-50 nm.
Another optical information recording medium which does not utilize interference effect as utilized in above literature is disclosed in Patent Literature 2. According to this known optical information recording medium, a fluorescent-light emission layer is provided under the recording layer, and the light emitted by the fluorescent-light emission layer is detected through the recording layer to read out the information.